A semiconductor memory device is a microelectronic device used in digital logic circuits such as microprocessors, which are in turn used in a wide range of electronic devices, from consumer electronic devices to satellites. Accordingly, the progress of technology for manufacturing highly integrated, high speed semiconductor memory devices is a key technology driver for increasing the performance of digital logic circuits.
Semiconductor memory devices may be classified as volatile memory devices or non-volatile memory devices. Data may be stored in a volatile memory device, and data stored in a volatile memory device can be read while electric power is supplied to the volatile memory device. However, data stored in a volatile memory device may be cleared when electric power is not supplied to the device. In contrast, a non-volatile memory device can continue to store data even when electric power is not being supplied to the device. Some types of non-volatile memory devices include mask read only memory (MROM), programmable read only memory (PROM), erasable and programmable ROM (EPROM), and/or electrically erasable and programmable ROM (EEPROM). Among the non-volatile memory devices, flash memory is widely used for computer and memory card storage since flash memory has the capability of simultaneously electrically erasing data stored in multiple cells of the memory.
Flash memory devices may be classified into NOR-type and NAND-type according to the type of connection between the cells and bit-lines. In NOR-type flash memory, more than two cell transistors may be coupled in parallel to one bit-line. A NOR-type flash memory stores data using hot electron injection and erases data using Fowler-Nordheim tunneling (F-N tunneling). In a NAND-type flash memory, more than two cell transistors may be coupled to one bit-line in serial. A NAND-type flash memory stores and erases data using F-N tunneling. Generally, the NOR-type flash memory configuration does not lend itself to high integration because NOR-type flash memory devices may consume large amounts of electric power. However, NOR-type flash memory devices may be advantageous for high speed operation. In contrast, NAND-type flash memory may be advantageous for high integration since NAND-type flash memory devices may consume less electric power than do NOR-type flash memory devices.
Methods for programming and/or erasing a NAND-type flash memory are disclosed, for example, in U.S. Pat. No. 5,473,563 entitled “Nonvolatile Semiconductor Memory” and U.S. Pat. No. 5,696,717 entitled “Nonvolatile Integrated Circuit Memory Devices Having Adjustable Erase/Program Threshold Voltage Verification Capability”, the disclosures of both of which are hereby incorporated herein by reference in their entirety. In order to program and erase a flash memory cell, a voltage higher than a supply voltage may be supplied to the cell. The voltage for programming and/or erasing a flash memory cell is referred to herein as a “program voltage.” A high/program voltage generating circuit for a flash memory is disclosed in U.S. Pat. No. 5,642,309 entitled “Auto-Program Circuit In A Nonvolatile Semiconductor Memory Device”, the disclosure of which is hereby incorporated herein by reference in its entirety.
FIG. 1 is a circuit diagram illustrating an array 110 of a conventional flash memory device, which may include a plurality of strings of memory cell floating gate transistors M0 to M15. Referring to FIG. 1, a flash memory generally includes an array 110 of memory cells, each of which may include a floating gate transistor. In a NAND-type flash memory, the array 110 may include strings (so-called “NAND strings”) of floating gate transistors. Each floating gate transistor M0 to M15 may be connected in serial between a string selection transistor SST and a ground selection transistor GST which are arranged in each string. Also, a plurality of word lines WL0 to WL15 are arranged to cross the NAND strings. Each word line WL0 to WL15 may be connected to a control gate of a corresponding floating gate transistor M0 to M15.
In an initial state, the floating gate transistors in the memory cells are cleared. In the cleared state, the floating gate transistors may have a threshold voltage of about −3V. In order to program a memory cell, a program voltage, for example 20V, may be supplied to a word line of a selected memory cell for a predetermined time period, which may be referred to herein as a program time or a programming interval. As a result, the threshold voltage of the selected memory cell may be raised to a higher threshold voltage. In contrast, the threshold voltages of non-selected memory cells are not raised.
However, some problems may arise when one or more of the memory cells are selected for programming among a plurality of memory cells connected to each other with same word line. For example, when a program voltage is supplied to a word line, the program voltage may be supplied not only to the selected memory cells, but also to non-selected memory cells which are connected to the same word line. As a result, the non-selected memory cells may also be programmed when the selected memory cells are programmed. This problem is referred to as a “program disturb” fault, which is an unintended programming of a non-selected memory cell connected to the selected word line.
In order to reduce program disturb faults, methods using a self-boosting scheme have been introduced. See, for example, U.S. Pat. No. 5,677,873 entitled “Method Of Programming Flash EEPROM Integrated Circuit Memory Devices To Prevent Inadvertent Programming Of Nondesignated Nand Memory Cells Therein” and U.S. Pat. No. 5,991,202 entitled “Method For Reducing Program Disturb During Self-Boosting In A NAND Flash Memory”, the disclosure of which is hereby incorporated herein by reference in its entirety.
In methods for reducing program disturb faults using a self-boosting scheme, a ground path may be cut off when 0V is supplied to a gate of a ground selection transistor. A voltage of 0V may be supplied to a selected bit-line, and a supply voltage (Vcc) of 3.3V or 5V may be supplied to a non-selected bit-line as a program inhibition voltage. At the same time, a power voltage may be supplied to the gate of a string selection transistor. After the source of the string selection transistor is charged up to Vcc-Vth, where Vth is a threshold voltage (Vth) of the string selection transistor, the string selection transistor is shut off. Then, a program voltage (Vpgm) may be supplied to the selected word line and a pass voltage (Vpass) may be supplied to the non-selected word lines for boosting the channel voltage of non-selected transistors. Accordingly, F-N tunneling may not be generated between a floating gate and a channel of a non-selected transistor. As a result, the non-selected transistors may be maintained in the initial clear state.
However, a coupling may be generated between adjacent word lines and adjacent signal lines SSL, GSL, if the rise time of the program voltage supplied to a word line is short (i.e., if the slope of the generated program voltage is large). In that case, the voltage supplied to the string selection line SSL or the ground selection line GSL may be momentarily raised. In particular, the coupling generated at the string selection line SSL may discharge the boosted channel charge through the string selection transistor SST. As a result, the boosting efficiency may decrease and a program disturb fault may occur. Therefore, there remains a demand for methods for controlling a program voltage to reduce the occurrence of program disturb faults, as well as methods for providing a controlled program voltage with a stable voltage level.